Basic Study On Dissimilar Metal Laser Welding Of TC4 And Inconel 718

In the advanced aeroengine,the titanium alloy and the superalloy are the two most widely used materials.The titanium alloy has excellent performance of small specific gravity,high specific strength,corrosion resistance and weldability.However,the heat resistance temperature of most titanium alloys are relatively low.Although the heat resistance temperature of the superalloys are higher,its density is greater,which will increase the engine weight and reduce the thrust-weight ratio.If the superalloy and the titanium alloy can be connected reliably,they will have broad application prospects in the field of aeroengine manufacturing.In this paper,Nd:YAG pulsed lasers and fiber lasers were used to perform laser lap welding experiments between TC4 and Inconel 718 with interchanged location.The influence of process parameters on the macroscopic morphology of the weld was analyzed.The observation of microstructure and defects,the analysis of tissue composition and hardness test were used by metallographic microscope,scanning electron microscopy,energy spectrometer and Vickers hardness tester respectively.During the YAG pulsed laser welding,with the increase of heat input,the weld surface quality became worth,and the penetration depth and weld width also increased.During the fiber laser welding,when the large defocussing and appropriate power and speed were used,the weld surface is free of spatter and crack.The melted top metal spread out the bottom plate smoothly.The most defects in the TC4/Inconel 718 joint are cracks,and pore defects are also observed.There are two kinds of cracks,cold crack and solidification crack.The gas of pores may resulted from hydrogen and water.The causes of the defects were analyzed and some measures to prevent the defects were put forward.The inside and the grain boundaries of the weld are mainly composed of 4 elements:Ti,Cr,Fe and Ni.Ti will generate intermetallic compounds with Cr,Fe,Ni.The weld microstructures are mostly mixed with the binary phases of Ti-Ni,Ti-Cr,Ti-Fe.The bond area is the most intense region of chemical composition.The degree of change of Ti element is the most obvious from the base metal to the weld.The hardness curve of the joint is trapezoidal,and the hardness of fusion zone is much larger than that of the two kinds of base materials.The hardness of fusion zone with an average of 824.7HV_1000g000g is the maximum in the case of using YAG pulse laser and Inconel 718 as top plate.While the hardness of fusion zone with an average of 619.5HV_1000g000g is the minimum in the case of using fiber laser and TC4 as top plate.The bond area can be divided into unmixed zone and partial melting zone.The microstructure of the partial melting zone in the TC4 side is a bulk phase,while that in the Inconel 718 side is mainly dendritic crystal.The microstructure of the unmixed zone is small and dense with a long strip or reticular distribution.Nearby the interface of bond area and fusion zone as well as the interface of bond area and base metal,the fluctuation of element content is most acute.In the middle of the partial melting zone,the chemical component fluctuates more stably when TC4 as bottom plate,while the element content shows a significant upward or downward trend when Inconel 718 as bottom plat.